1. Field of the Invention
The invention disclosed herein relates to investigations of an earth formation penetrated by a borehole having non-conductive mud and, more specifically, to accurately measuring the resistivity of the earth formation and providing an accurate image from the measuring.
2. Description of the Related Art
A borehole is typically drilled into a geologic formation for various applications such as carbon sequestration, geothermal production, and hydrocarbon exploration and production. In order to efficiently use expensive drilling resources, it is important to acquire detailed information concerning the properties of the geologic formation.
A logging or downhole tool can be used to acquire this information. The tool is disposed in the borehole by wireline for wireline logging or in a collar attached to a drill string for logging-while-drilling applications. Many different types of tools can be used to measure different properties of the formation.
One type of tool is a galvanic logging tool used to measure the resistivity of the formation as a function of depth in the borehole. Variations in the measured resistivity can be plotted to produce a resistivity image of the formation. In order to measure the resistivity, the galvanic logging tool injects alternating electric current into the formation using an electrode. The electric current is then received by another electrode coupled to apparatus for measuring the electrical characteristics of the received current. The active part of the impedance of the formation to the flow of current is primarily related to the resistivity of the formation. Hence, measured electrical characteristics of the received current can be used to determine the resistivity of the formation.
If any of the electrodes are not in direct contact with the wall of the formation (i.e., borehole wall) due to borehole rugosity, for example, the electric current must cross a gap, referred to as “standoff,” between the corresponding electrode and the borehole wall. When a borehole is drilled through water-soluble formations, oil-based drilling muds must be used for lubricating and cooling the drill bit and flushing the cuttings from the borehole. An oil-based drilling mud is non-conductive and, thus, can further impede the electric current and reduce the accuracy of conventional resistivity imaging devices. Improving galvanic resistivity imaging in a non-conducting mud environment would be well received in the art.